In the semiconductor arts multi-layer structures are created upon semiconductor wafers that comprise electrically conductive lines and vias surrounded by one or more electrically insulating dielectric layers. The lines, vias and dielectric layers may be said to be “pattered” into specific structures that implement specific circuitry. Because the dimensions of these patterns can be extremely small (e.g., current mass production technologies can reduce features to as small as 90 nm across), the impact of a manufacturing defect is becoming more important as is the difficulty in detecting it.
In order to detect manufacturing defects optically, a spot of light is focused on a region of the wafer's patterned structures. The reflected image of the patterned structure is captured through an optical channel and resolved on an optical detection device such as a photodetector or charge coupled device (CCD). Data produced by the optical detection device is then compared against “expected data”; where, the expected data corresponds to the data from the optical detection device if the manufactured patterns illuminated by the focused spot of light were properly manufactured.
FIGS. 1a and 1b demonstrate a problem that exists with respect to the optical detection of a manufacturing defect. FIG. 1a shows an example of a “noiseless” optical signal that is resolved to the optical detection device; and, FIG. 1b shows an example of a “noisy” optical signal that is resolved to the optical detection device. Spot 101a corresponds to the illuminated spot of light that is focused on the wafer's structural patterns. As a simple example, the wafer's structural patterns are shown to include a repeating pattern of conductive lines 1021 through 1025 separated by a dielectric material region (which is drawn in FIG. 1a as a shaded region).
Looking along axis 104, a one-dimensional signal 107 should be resolved upon the optical detection device. Assuming the conductive lines have higher reflectivity of the focused spot of light than the dielectric material, enhanced optical intensity should be observed at the detector for the conductive lines as compared to the dielectric regions. Signal 107 indicates as much through optical intensity spikes 1051 through 1055 which are meant to correspond to conductive lines 1021 through 1025, respectively.
A manufacturing defect 103 is also observed in the image 101a of FIG. 1a. This manufacturing defect may be the result of a thin layer of “spilled” conductive material, a “pit” or “void”, etc. Whatever its form, the defect ultimately reproduces on the optical detection device as part of signal 107 with an intensity 106 that is less than the intensity of the spikes 1051 through 1055 that are associated with the conductive lines 1021 through 1025.
FIG. 1b shows an image 101b of the same region of the same wafer as depicted in FIG. 1a, but with noise attributed to complications that arise from the optical processing of the reflected image from the wafer. Specifically, because the patterned structures on the wafer are “three dimensional” in the sense that the conductive lines 1021 through 1025 have “edges” that determine the lines' thickness, the light that reflects off of the wafer does not reflect uniformly off of the wafer surface. For example, a ray of light that impinges directly upon a flat portion of a conductive line may reflect perpendicular to the surface of the wafer while a ray of light that impinges at an edge of a conductive line may reflect at an angle other than perpendicular with the wafer surface (i.e., the angle of reflection is different for rays that impinge upon flat conductive line surfaces as opposed to conductive line edges).
As processed by the optical channel between the wafer and the optical detection device, the various components of light that reflect off of the wafer at varying angles depending on surface topography (and intensity depending on reflectivity) may constructively or de-constructively interfere so as to create bright or dark “noise” spots in the signal that is resolved on the optical detection device. FIG. 1b shows such a bright noise spot 108 (also referred to as a “lobe”) that might result, for instance, from the constructive interference of light reflected off of the edges of the conductive lines.
The portion 111 of the resolved signal 110 that corresponds to the lobe 108, being a deviation from the noiseless signal 107, corresponds to an item of noise that makes detection of the defect difficult or impossible. For example, with the lobe 108 being positioned around the defect itself, the resolved signal superimposes the intensity spikes 112, 113 that correspond to conductive line 1024 and defect 103, respectively. At a minimum, owing to the intensity 111 of the lobe, it will more difficult to detect the signal from the defect 113 in FIG. 1b as compared to the signal from defect 106 observed in FIG. 1a. In an extreme case, if the lobe's intensity is beyond the saturation level 114 of the optical detector device, the defect will be impossible to detect (because the data from the optical detection device will clip at level 114).